1. Field of the Invention
The present invention relates generally to electrochemical electrodes and, more particularly, to combination electrodes for measuring the ion concentration of solutions.
2. Description of the Prior Art
Combination electrodes comprising a sensing portion and a reference portion are well known in the art. One known electrode assembly for measuring pH includes a tubular, glass pH electrode coaxially supported within a durable, tubular, plastic container to define an annular reference electrolyte reservoir between the electrode body and the container. In such a structure it is necessary to establish electrolytic contact between a reference electrolyte within the annular reservoir and a test solution into which the electrode is immersed. Typically this is accomplished by means of a wettable material, such as asbestos or linen fibers, which provides a minute flow rate leakage path or liquid junction between the electrolyte and the test solution. For example, the leakage path of one commercial combination electrode assembly, illustrated in copending application Ser. No. 629,833, (now U.S. Pat. No. 4,012,308) filed Nov. 7, 1975, and assigned to the assignee of the present invention, comprises a plurality of asbestos fibers spaced around and extending through an annular seal between an inner glass pH electrode body and an outer plastic container.
While asbestos fiber liquid junction structures generally function satisfactorily, they have several drawbacks and limitations. First, they require tedious operations by hand to incorporate in an electrode assembly. Second, they do not readily lend themselves to the formation of an annular liquid junction as would be desirable in combination electrodes having annular electrolyte reservoirs. Third, they can easily become clogged during use.
U.S. Pat. No. 3,492,216 (Riseman et al.) proposes a combination, ion-selective electrode assembly incorporating an annular liquid junction structure without the use of asbestos fibers or other liquid junction materials. In this regard, the electrode aseembly includes inner and outer tubular plastic bodies. The exterior surface at one end of the inner body is outwardly tapered as a frustoconical end portion. The adjacent interior surface of the outer body is conically tapered at the same angle as the frustoconical end portion so that the inner body will nest within the outer body with the tapered surfaces mating with each other and the ends of the bodies defining a planar sensing surface of the electrodes assembly. The mating tapered surfaces are roughened to establish an annular leakage path between an electrolyte reservoir within the electrode assembly and a test solution into which the assembly is to be immersed.
A similar precision-fit, annular leakage junction is described in U.S. Pat. No. 2,058,761 (Beckman et al.) for a single reference electrode and comprises a frustoconical plug seated within a glass tube to establish an annular leakage path through the contact area between the plug and the tube.
In both of the foregoing approaches, precision grinding and preparation of the mating tapered surfaces is required to establish the perfect surface area contact therebetween. Obviously, it would be desirable to provide an annular leakage path for a combination electrode in a manner which eliminates the precision manufacturing steps heretofore employed without sacrificing the durability and reliability of the electrode. The present invention meets these needs.